It’s Not About the Wheels

Recently I read a forum posting in which the writer stated she preferred 700c wheels to 650c wheels because she sat higher on the bike and felt more on a par with riders around her. She may have felt this way, but it had nothing to do with wheel size. Something else was going on.

On first take, you might think the larger the wheel, the higher the bike. Seems logical, but it’s not. Whether a bike is being designed around 700c, 650c, 650b, 24″ or 20″ wheels, there’s one dimension that usually doesn’t change much — the bottom bracket height. It’s the distance from the ground to the center of the bottom bracket, the “central movement” according to the Italians. It’s where the crank arms meet the bicycle.

So why is the bottom bracket height important? Well, if it’s too low, you’ll scrape your downside pedal when you’re cornering. Not a safe situation. If you have really short crank arms on your bike, the bottom bracket height can be lower than it would be on a bike with long crank arms. If the bottom bracket height is too high, your center of gravity will be high and the bike won’t feel stable in turns. Mountain bikes used to be built with very high bottom bracket heights to clear debris on the trail.

Back to our rider. Assuming she had the same fit on both bikes, then she was using the same crank arm length on both bikes and therefore, the distance from the pedal to the top of her saddle was the same. If the seat angle was the same, then the distance from the ground to the top of the saddle was identical on both bikes. So her hips were in the same place on both bikes.

How about her upper body? Again, if the “cockpit” fit identically on both bikes, her reach to the handlebars was the same and the vertical distance between the seat and the handlebars was the same. So her upper body was in the same position. In other words, she was exactly the same rider in exactly the same height/position on both bikes.

Here are two drawings showing our hypothetical rider on bikes with 650c and 700c wheels. The distance to the saddle from the pedals is identical as are the “cockpit” dimensions. Two drawings are worth six words: “it’s not about the wheel size.”

So why does this rider feel taller on the 700c bike? Well, one reason might have to do with the seat angle on the bike. If the 700c bike has a steeper seat angle than the 650c bike, she will ride higher in the saddle. Imagine a 90 degree seat angle: she would be sitting really high! There are a lot of 700c bikes for women built with steep seat angles in order to make the top tubes shorter, so that might be one of these bikes. (Not the right way to do it, by the way…)

Or perhaps she’s riding with different “cockpit” dimensions. If the reach to the bars is shorter, the stem is shorter and/or the stem is taller, she will sit more upright. Taller in the saddle. And back to the bottom bracket height — it may indeed be higher on the 700c bike.

Lots to think about here. One of the most important is that wheel size is one of many tools available to bike designers so we can give our riders the best fit possible. The bike is a wonderful work of engineering and art whose pieces are remarkably intertwined — as much with each other as they are with you, the cyclist.